Electroerosion printing is a well-known technique for producing markings, such as letters, numbers, symbols, patterns, such as circuit patterns, or other legible or coded indicia on recording material in response to an electric signal which removes or erodes material from the surface of the recording material as the result of spark initiation (arcing).
The surface which is electrically eroded or removed to provided such indicia on the recording material is usually a thin film of conductive material which is vaporized in response to localized heating associated with sparking (arcing) initiated by applying an electric current to an electrode in contact with the surface of a recording material comprising the thin conductive film on a non-conductive backing or support. In the present state of the technology, the thin conductive film is usually a thin film of vaporizable metal, such as aluminum. In practice, a multi-styli print head is scanned across the aluminum surface of the recording medium and electrodes are selectively energized to form images in accordance with digitally coded image information. Electroerosion materials and processes are useful to produce directly, human readable images, photomasks, etc. For a number of such applications, flexible substrates of paper and plastic have been employed with thickness on the order of 2 to 5 mil with vapor deposited aluminum films of a thickness on the order of 0.5 to 5 micrometers having been utilized. For details on materials heretofore used in electroerosion printing, see U.S. Pat. No. 4,082,902, Suzuki, and U.S. Pat. No. 4,086,853, Figov.
In high resolution and high speed electroerosion printing, the print head used to record the desired information may be comprised of thirty or more styli which move in relation to the surface of specially prepared recording media. Electrical writing signals are fed to the stylus or styli to provide controlled electrical pulses which generate arcing at the surface of the recording material selectively to heat and remove by evaporation a layer, usually aluminum, of the recording material; the removed material corresponds to the indicia which are to be recorded.
A common problem with high resolution electroerosion printing processes using conventional metallized plastic substrates is that severe scratching of the aluminum layer occurs during writing. This may be attributable to the relatively low resistance of a thin aluminum film to mechanical abrasion or to other causes, including plastic deformation of the substrate. The thin conductive film, for example, the vapor-deposited thin aluminum film, apparently cannot withstand the high strains generated when the support or substrate is deformed, resulting in scratching. Also, in the instance that the styli cold-weld to the thin aluminum conductive layer, the structure may suffer shear, either at the aluminum-substrate interface or below it in the substrate itself.
Various measures have been proposed to combat the problem of scratching. For example, it has been suggested to suppress the plastic deformation of the substrate by creating a thin hard layer below the aluminum. The hardness of the layer would be selected to prevent the undesired deformation, but the thickness of the layer would be such that the gross flexibility of the sheet necessary for electroerosion writing and handling would be maintained. The method of obtaining this hard layer involves the incorporation of small hard particles, such as silica, in a suitable polymeric binder.
This system was found in fact to decrease scratching caused by the polymer-substrate deformation mechanism. Although such a base layer between the plastic substrate and the aluminum film provides substantial improvement in scratch resistance, some scratching still remains, caused by the fact that the hard particles in these coatings are easily abraded during the printing process due to high local pressure caused by the styli on such rough surfaces.
In the co-pending application Ser. No. 454,743, filed Dec. 30, 1982, expressly incorporated herein by reference, there is disclosed electroerosion recording materials comprising a non-conductive support, a thin layer of conductive material capable of being removed by evaporation during electroerosion recording and an improved intermediate layer of thin, hard cross-linked polymer between said support and said layer of conductive material. The cross-linked polymer layer, such as a thermoset cellulose-acetate-butyrate polymer (CAB) cross-linked with polyisocyanate, and containing hard particles such as silica, is found to reduce plastic deformation of the support in response to stylus writing pressure and thus minimizes scratching during the electroerosion process, while the surface roughness is sufficient to scour off from the stylus debris formed during electroerosion recording.
However, with the employment of either a noncross-linked polymer based or the above-described cross-linked polymer based intermediate layer, it has been necessary to include a lubricant overcoat to further improve the scratch-resistance and print quality of the electroerosion material. Lubricant coatings containing conductive particles of high lubricity, such as graphite, in a polymeric binder are disclosed in copending application Ser. No. 454,744, filed Dec. 30, 1982.
Although with the application of a lubricant overlayer and a relatively hard cross-linked base layer, it has been possible to obtain much better scratch resistance, further modifications of the chemistry of various coatings are desired for an overall improvement so as to obtain reduced head wear, and reduced "fouling" or "baking" problems which are caused by the accumulation of eroded debris on the print head during the writing process.
U.S. Pat. No. 3,740,254, Lansbury et al, describes an isocyanate-ended polyurethane composition, i.e., a prepolymer, as a primer for the deposition of thin aluminum films on various substates including plastic substrates. The purpose here is to improve adhesion of the aluminum film.
U.S. Pat. No. 3,786,518, Atherton, describes an electroerosion recording material comprising, for example, a thin conductive film of aluminum deposited over a resin which has been provided with a matte finish by the inclusion of a matting agent or by treating the resin surface. A wide variety of resinous materials is suggested for the resin layer.
U.S. Pat. No. 4,339,758 of Bhatia et al, describes an electrosensitive recording that uses a silicon dioxide containing resinous base layer between the support and the overlying metallic film.